Study On Discharge Characteristics Of Oil-paper Insulation Under High Frequency Pulse Voltage

With the development of new energy generation and HVDC transmission,high frequency transformers have received extensive attention.Compared with traditional power frequency transformers,high frequency transformers have the advantages of small volume,light weight and adjustable output frequency,so it is easy to realize the functions of new energy grid connection and AC/DC transfer.Large capacity high frequency transformer plays an important role in electrical isolation and power transmission,but the problem of insulation and heat dissipation limits the development of high frequency transformer to large capacity and high frequency.Oil paper insulation has good insulation performance and heat dissipation performance at the same time.Compared with dry insulation,it is more suitable for of high frequency transformer.However,because the high frequency transformer runs in the nonsinusoidal voltage with short rise time,large amplitude and high frequency for a long time,the performance change of oil insulation paper at high frequency voltage will seriously affect the overall insulation performance of high frequency transformer.At the same time,the insulation problems caused by transient high frequency overvoltage in traditional oil-paper insulated transformers are also prominent.Therefore,it is of great significance to study the insulating discharge characteristics of oil paper at high frequency voltage.A high frequency oil paper insulated partial discharge experimental platform is built in this paper.the high frequency bipolar square wave pulse power supply is used to simulate the high frequency electrical stress in high frequency transformer.the detection impedance method and high frequency CT method are used to collect the partial discharge signal.the maximum discharge quantity,average discharge quantity,discharge repetition rate,discharge current,discharge power and discharge phase spectrum are selected to describe the partial discharge characteristics under high frequency voltage.Based on the high frequency partial discharge test platform,the oil paper insulation partial discharge experiment under different voltage frequency(1kHz-20 kHz)was carried out by using needle plate electrode model.The partial discharge activity at high frequency square wave voltage mainly occurs at the rising and falling edges of polarity reversal,while the partial discharge activity in flat top area is less.At the same time,with the increase of voltage frequency,the discharge statistics such as discharge quantity,discharge current and discharge power increase first and then decrease,and there is a frequency inflection point near 10 kHz.Compared with partial discharge at power frequency voltage,it is found that the initial voltage of partial discharge of oil paper insulation is lower than that of power frequency,and the discharge intensity is much higher than that of partial discharge at power frequency.The breakdown stage is more likely to occur direct strike phenomenon.The above phenomena are related to the high frequency heat effect and the effect of frequency on the charge dissipation time.In this paper,the breakdown type of oil-paper insulation and the damage mechanism of discharge on oil-paper insulation were studied by means of micro-morphology analysis and physical and chemical properties test.The results show that the edge of the breakdown point of oil-paper insulation is bulging and the section structure is regular under high frequency voltage.Fourier infrared spectra of the damaged area of insulator paper showed that discharge changed the number and structure of functional groups on the surface of insulator paper,the molecular chain of aramid fiber was broken,and the intermolecular force was reduced.At the same time,discharge promoted the hydrolysis and cracking of Nomex paper.Under the action of high frequency partial discharge,the crystal type of Nomex insulator paper and the coexistence structure of crystal phase and amorphous phase have not changed,but the relative crystallinity of insulator paper increases,but the partial discharge at power frequency has little effect on the aggregation structure of Nomex insulator paper.